This invention relates to capacitive relative-pressure sensors. With such sensors, pressures of media, such as liquids, gases, or vapors, can be measured, the measurement being made in relation to the current atmospheric or ambient pressure, which thus serves as a reference pressure.
U.S. Pat. No. 5,079,953 discloses, as one of three variants, a capacitive ceramic relative-pressure sensor comprising
a diaphragm
having a surface on which a first electrode is deposited, and
a substrate
having a bore for guiding reference air from a first surface to an opposite, second surface,
on which at least a second electrode is deposited,
said substrate and said diaphragm being soldered or brazed together around the periphery by means of a spacer to form a chamber.
In such relative-pressure sensors, referred to herein as xe2x80x9cpressure sensorsxe2x80x9d for simplicity, the so-called reference air from the atmospheric air, referred to herein as xe2x80x9cambient airxe2x80x9d, which, of course, is always more or less humid, flows through the bore in the substrate into the chamber.
By a suitable design of the housing for the pressure sensor, if the ambient air is saturated with humidity, the ambient air, before entering the chamber, can be caused to flow through or over a point having the temperature of its dew point, so that thereafter the temperature of the reference air will not pass below the dew point; this is accomplished by forcing the ambient air to pass a point of the housing whose temperature is equal to or less than that of the chamber in the sensor, so that the dew point of the ambient air is reached, i.e., condensation occurs, already at that point, and only reference air not saturated or oversaturated with humidity will reach the chamber. The relative humidity of the reference air may therefore be high, e.g., up to 95%.
Such a high relative humidity in the chamber of the pressure sensor causes the following problems, which are associated with the way in which capacitive ceramic pressure sensors are commonly manufactured.
Blanks for the diaphragm and substrate are so-called green compacts, which are preformed from a powdered ceramic starting material and a binder and subsequently sin-tered.
The starting ceramic material may be alumina, for example. The sintered alumina compacts of pressure sensors, i.e., the respective substrates and diaphragms of the sensors, generally have a purity of 96 wt. %. For special applications, however, the purity may be up to 99.9 wt. %. The sintered compacts have not only rough surfaces but also microcracks extending from these surfaces into the sintered compacts.
To produce a pressure sensor from a sintered diaphragm compact and a sintered substrate compact, the substrate and the diaphragm are soldered or brazed together along the periphery with the interposition of a spacer, so that the aforementioned chamber is formed. The solder or active brazing solder used represents the spacer. The solder is, for example, a glass frit, and the active brazing solder is, for example, an NiTiZr alloy in which the NiZr content is approximately equal to the NiZr eutectic, see U.S. Pat. No. 5,334,344. The soldering or brazing is also referred to as xe2x80x9cjoiningxe2x80x9d.
Prior to the joining of the diaphragm and the substrate, electrodes are deposited on the surfaces that will face each other in the chamber after the joining. These electrodes are made of tantalum, for example, see U.S. Pat. No. 5,050,034, and are commonly deposited by sputtering. If the diaphragm and the substrate are joined using active brazing solder, the electrode or electrodes on the substrate must be located at a distance, i.e., be electrically isolated, from the joining material, since in the finished pressure sensor, the electrode of the diaphragm is electrically connected to the joining material.
The electrode or electrodes of the substrate must therefore be sputtered through a mask that is fixed directly on the surface of the substrate and covers that portion of the surface which has to be kept free of electrode material.
During the sputtering it is unavoidable, however, that due to so-called undersputtering, minute islands of electrode material, which are not in electrical contact with each other, are formed on the surface to be kept free of electrode material. Furthermore, as a result of the undersputtering, electrode material can penetrate into the above-mentioned microcracks.
Since water molecules of the reference air can both be adsorbed by the rough surface and penetrate into the microcracks, electrical connections are formed between the individual islands of electrode material, so that the area of the substrate electrode increases. This, however, results in a zero offset of the pressure sensor. This effect increases with increasing relative humidity of the reference air.
The problems described so far have hitherto been controlled relatively well by applying to the finished electrodes a spin-on glass layer of silicon dioxide, which actually serves to eliminate another problem, see U.S. Pat. No. 5,400,489. In this way, however, only relative humidities up to about 80% are controllable. Thus, at a relative humidity of 80%, related to 400xc2x0 C., such pressure sensors have a zero offset of up to 1%. For high-accuracy measurements, this is not acceptable.
It is therefore an object of the invention to provide relative-pressure sensors that have virtually no zero offset at relative humidities up to near the saturation limit.
To attain this object, the invention provides a capacitive ceramic relative-pressure sensor comprising:
a diaphragm
having a surface on which a first electrode is deposited; and
a substrate
having a bore for guiding reference air from a first surface to an opposite, second surface,
the first surface of the substrate being polished and provided with at least a second electrode,
which substrate and which diaphragm are soldered or brazed together along the periphery by means of a spacer to form a chamber,
which chamber is covered from inside with a thin layer of hydrophobic material that is introduced through the bore after the soldering or brazing.
In a preferred embodiment of the invention, the hydrophobic material is a silicone oil, a paraffin oil, or a silicone resin based on fluorinated siloxanes or on methyl polysiloxanes.
In another preferred embodiment of the invention, the substrate and the diaphragm are made of alumina ceramic.
One advantage of the relative-pressure sensors according to the invention is that they have a zero offset less than 0.2% even at a relative humidity of 95%, related to 400xc2x0 C.